linux-kernel - Re: [RFC V2 2/2] sched: idle: IRQ based next prediction for idle period

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Date:	Wed, 20 Jan 2016 14:17:57 -0500 (EST)
From:	Nicolas Pitre <nicolas.pitre@...aro.org>
To:	Peter Zijlstra <peterz@...radead.org>
cc:	Daniel Lezcano <daniel.lezcano@...aro.org>, tglx@...utronix.de,
	rafael@...nel.org, linux-pm@...r.kernel.org,
	linux-kernel@...r.kernel.org, vincent.guittot@...aro.org
Subject: Re: [RFC V2 2/2] sched: idle: IRQ based next prediction for idle
 period

On Wed, 20 Jan 2016, Peter Zijlstra wrote:

> On Wed, Jan 20, 2016 at 05:00:33PM +0100, Daniel Lezcano wrote:
> > +static void sched_irq_timing_handler(unsigned int irq, ktime_t timestamp, void *dev_id)
> > +{
> > +	u32 diff;
> > +	unsigned int cpu = raw_smp_processor_id();
> > +	struct wakeup *w = per_cpu(wakeups[irq], cpu);
> > +
> > +	/*
> > +	 * It is the first time the interrupt occurs of the series, we
> > +	 * can't do any stats as we don't have an interval, just store
> > +	 * the timestamp and exit.
> > +	 */
> > +	if (ktime_equal(w->timestamp, ktime_set(0, 0))) {
> > +		w->timestamp = timestamp;
> > +		return;
> > +	}
> > +
> > +	/*
> > +	 * Microsec resolution is enough for our purpose.
> > +	 */
> 
> It is also a friggin pointless /1000. The cpuidle code also loves to do
> this, and its silly, u64 add/sub are _way_ cheaper than u64 / 1000.

For the purpose of this code, nanoseconds simply provides too many bits 
for what we care.  Computing the variance implies squared values.

*However* we can simply do diff = (timestamp - w->timestamp) >> 10 
instead.  No need to have an exact microsecs base.

> > +	ktime_t now = ktime_get();
> 
> Why !?! do we care about NTP correct timestamps?

Not at all. Using sched_clock() instead would be more than good enough 
indeed.


Nicolas